Proposals have already been made in patents FR 2 736 149 and U.S. Pat. No. 5,762,292 for a system for recognizing structures presenting rectilinear portions in an image delivered by a sensor on board a flying object, by using a method that implements a parametric transform (Hough transform) of a portion of the image.
The Hough transform, which is described in U.S. Pat. No. 3,069,654, enables a set of aligned points to be detected in an image.
Proposals are made in U.S. Pat. No. 5,296,909 to detect the presence of cables by means of a scanning laser telemeter (Lidar) that delivers echoes or plots, where each plot corresponds to a point in three-dimensional space that is characterized by its three coordinates in that space: the telemeter emits laser pulses that, by measuring their travel times, enable points to be obtained that are positioned in 3D space (3D plots). The echoes are filtered; a set of parameters is determined by the Hough transform for all possible groups of filtered echoes; accumulations of points in the parameter space are identified, and the position of a cable is determined by an inverse Hough transform.
Proposals are also made in U.S. Pat. No. 6,747,576 to detect the presence of electrical power lines by forming a cloud of measurement points in a terrestrial coordinate system on the basis of data delivered by a remote detection sensor and data delivered by a navigation system, and by eliminating measurement points that represent the ground; the method then comprises searching for straight lines amongst the projections of the measurement points onto a horizontal plane, by performing two successive Hough transforms: a “pure” transform using a delta function (Dirac function), followed by a “fuzzy” transform where the delta function is replaced by a Gaussian distribution; thereafter a search is made for catenaries in each vertical plane containing one of the straight lines as found in that way, said search likewise comprising two successive Hough transforms.
In order to search for a catenary corresponding to the equation:z=a*cos h((λ−b)/a)+cfor each measurement point in each vertical plane, and for each possible value of a catenary parameter a, a two-dimensional Hough transform of the catenaries passing through said point is calculated (in the b and c parameter space).
That method requires very considerable computation time because of the three-dimensional nature of the space to be searched, and it also requires a specific accelerator card, even when selecting a large digitization step and small ranges for the catenary parameters, to the detriment of result accuracy.
Although certain cable detection systems have already been described, at present there does not exist any system that is effective in bad weather conditions.
Present detector systems also lack performance in clear weather: when the sighting angle is large, a telemeter detects only a few 3D plots for the complete structure of a cable; in practice, it is then not possible to determine the presence of a cable, nor to situate it in three-dimensional space.